This invention relates to synthetic quartz crystals. In a more particular manner, this invention concerns itself with the growth-of substantially dislocation-free synthetic quartz crystals produced from substantially dislocation-free quartz seed crystals.
Quartz, chemically composed of silicon-dioxide and trace amounts of other materials such as aluminum and sodium and lithium alkalies, is one of the most abundant minerals in our geological environment. It often occurs in crystalline form and, most often, is colorless and transluscent. It's crystallized structure is either right-handed or left-handed and rotates the plane of polarization of transmitted light. One of its properties, that make it especially useful for a wide variety of industrial and military applications, is its piezoelectricity. That is, the crystal is capable of producing an electric charge on some of its surfaces when the crystal is ccmpressed in certain directions. The charge disappears when the compression is removed. This particular property makes quartz crystals especially useful in microphones, phonograph pickups, ultrasonic generators and in electromechanical devices, such as freqency-controlling quartz crystal resonators. These quartz crystal resonators are widely used in military and commercial communication systems for carrier-frequency separation and for selecting a desired frequency signal while rejecting undesired frequencies.
Generally, the piezoelectric quartz crystals used for industrial and military applications are synthetic in nature and grown commercially using a number of conventional crystal growing techniques. One of the more successful processes employed universally for synthetic quartz growth is the so-called hydrothermal technique. It is similar in theory to the well known solution growth technique and involves growing quartz crystals under high temperatures and pressures. Nutrient, or feed material, is dissolved in a high temperature region of a vessel. The solution is transported by thermal gradients to a lower temperature region, becomes supersaturated and the material precipitates on a quartz crystal seed in single crystal form. The temperatures involved in the hydrothermal process are higher than in solution growth and pressures of 1000 atmospheres or more may be used.
In the hydrothermal process, quartz growth takes place usually in a high pressure autoclave. The interior diameter of the autoclave is between 25-35 cm and the height between 3-5 meters. The nutrient, usually natural quartz chips, also called lascas, is placed in the bottom of the autoclave and separated from the top by a baffle containing small holes. Production autoclaves often use from 100-150 kg of nutrient per run. The quartz crystal seeds are suspended on racks positioned in the upper region of the autoclave which is at a lower temperature than the bottom. Typically, there are over 100 seeds in the autoclave. Thermal insulation is placed around the outside of the autoclave. All vessels used commercially are fabricated from steel and these are often buried in the ground or enclosed in heavy metal sheets as a safety factor.
Three crucial growth parameters, pressure, temperature, and fill are interrelated in the hydrothermal process, pressure being a function of temperature and fill. The fill, i.e., the percentage of the vessel volume filled with solid and liquid prior to sealing, is generally about 80 percent. At growth temperatures, owing to thermal expansion of water, the vessel is almost entirely filled with liquid. Fill is the most important parameter determining the average autoclave pressure. A ten percent change in fill will increase the pressure more than twofold at 350.degree. C. The fill for quartz has been varied from 65 to 88 percent corresponding to pressures of 150 to 3000 atmospheres. Most present commercial growth processes use up to 2000 atmospheres pressure.
The effect of temperature on pressure is less significant than fill, but temperature must be carefully controlled to prevent the pressure from exceeding autoclave mechanical design limits. Catastrophic failures have occurred even at commercial establishments. For alpha-quartz, the growth temperature T.sub.g, near the seeds, can be varied from 200.degree. to 573.degree. C. At 200.degree. the reaction is very slow but above 573.degree. C. undesirable beta-quartz is formed. Typical growth temperatures vary from 340.degree. to 375.degree. C.
The temperature differential T, between the two sections of the autoclave, the top part containing the seeds and the bottom part containing the nutrient, is significant to growth rate and can be varied between 5.degree. and 100.degree. C. In general however, the differential depends on the growth process and desired grade. More uniform crystals are obtained with smaller temperature differentials. The proper temperature differential, profile, and schedule for constant growth rate is usually established empirically for each autoclave.
The solubility of quartz in water under hydrothermal conditions ranges from 0.1 to 0.5 weight percent. These values can be increased to the desirable level of 2-5 percent by the addition of a mineralizer solution to the nutrient solution. The mineralizer solution can be either sodium hydroxide or sodium carbonate in the range of 0.5 to 1 normal. The growth rate of quartz is slow compared to most single crystal techniques being in the range of 20 to 40 mil/day. Thus, to produce material of the desired size, growth runs can extend from 25 to over a hundred days.
The seed crystal used in hydrothermal growth is generally either a zero degree seed, correponding to the z face of the crystal, or a 5.degree. seed, representing a plane 5.degree. from the zero face. One of the major drawbacks of the hydrothermal process, as well as other crystal growing techniques which utilize seed crystals to propagate crystal growth, is the propagation of defects in the growing of quartz crystals boules. As stated heretofore, synthetic quartz crystals are generally grown using a seed cut perpendicular or nearly perpendicular to the Z-axis crystallographic orientation. Unfortunately, the seed crystals presently employed contain various defects which are detected by x-ray topography. These defects in the seed result in the production of dislocations during crystal growth which propagate along the axis of growth of the growing crystal. When seed crystals are then sliced from the grown crystal, the dislocations are again propagated in another growing crystal resulting in the continuing propagation of the dislocations in future runs.
Efforts are presently underway in an attempt to reduce the number of dislocations in the grown crystal by reducing the number of defects in the seed. This has been done by using select natural crystals, but it would be more economical if a technique could be found using synthetic material. Unfortunately, these previous efforts have not been too successful. With this invention, however, it has been found that quartz crystal seeds cut from the x-plus region of a quartz crystal boule can be used as seed crystals to grow substantially dislocation-free quartz. The dislocation-free quartz is especialy valuable and useful for precision timing and frequency devices.